1. Field of the Invention
The present invention relates to metal porous bodies comprising an alloy having high strength, excellent corrosion resistance and heat resistance, which have applications in electrode substrates, catalyst supports, filters, metallic composite materials and the like, and also relates to a method of preparing such metal porous bodies.
2. Description of the Related Art
Conventionally, metal porous bodies have been used in various applications such as filters and battery plates where heat resistance is required, and catalyst supports and metallic composite materials. Hence, techniques for the preparation of metal porous bodies have come to be known through a variety of documents. Furthermore, products in which an Ni-based metal porous body commercially available as xe2x80x9cCELMETxe2x80x9d (registered trade name) prepared by Sumitomo Electric Ind., Ltd. are already being widely used in industry.
As conventionally known methods for the preparation of metal porous bodies, there are the plating method which is performed after treating a foamed resin or the like to render the same to be electrically conductive as disclosed in Japanese Patent Application Laid-Open No. 57-174484, and the method in which metal powder is formed into a slurry, the slurry is applied to a foamed resin or the like, and sintered, as disclosed in Japanese Patent Publication No. S38-17554.
The plating method involves attachment of an electrically conductive material, vapor deposition of an electrically conductive material, or surface modification with a chemical agent, as a treatment for rendering the surface of a foamed resin or the like electrically conductive. A metal porous body is obtained by metal plating the foamed resin or the like which has been rendered electrically conductive, and then burning out and removing the resin part therefrom. Electro-plating and electroless plating, for example, can be used in the formation of the metal skeleton. However, since the both methods involve plating, a metal porous body thus obtained consists of a single metal in either case. Known alloying treatments include a method in which, after the plating with different types of metal, these metals are diffused in a later step, and a method in which, after the plating with a single metal, diffusion alloying treatment is performed.
In the sintering method, a slurry comprising metal powder and resin is coated or sprayed onto a foamed resin or the like, and then subjected to a sintering treatment after drying. With the method disclosed in the aforementioned Japanese Patent Publication No. 38-17554, alloying treatment can be performed if several types of metal materials are used.
However, although it is possible to obtain alloyed metal porous bodies, they are inferior in strength to the metal porous bodies obtained by a combination of plating and diffusion alloying treatments. This problem is related due to adhesion among the metal powders obtained by sintering.
As a means of improvement in this respect, Japanese Patent Publication No. 6-89376 discloses a method for improving the adhesion in which the surface of iron powder is oxidized while controlling the carbon content in the iron powder, so that the surface of iron is reduced during sintering as a result of an oxidation-reduction reaction between the carbon contained in the iron and the oxygen in the oxide formed on the iron surface. With this method, however, the metal parts within the iron particles take no part in the reaction. Therefore, although it provides an improvement at the boundary faces in the resultant skeleton structure, the inadequacy of the mechanical strength in the original metal structure still remains.
Furthermore, Japanese Patent Application Laid Open No. 9-231983 discloses fine-grained metal sintered porous bodies which are produced from iron oxide powder as a raw material. Since metal porous bodies consisting of iron alone are inadequate in terms of strength, corrosion resistance and heat resistance, these properties are improved by alloying in this disclosure. However, the alloying of the aforementioned invention is not realized simply by adding powder or an oxide of a metal other than iron.
Moreover, there is a trend that metal porous bodies are used more and more for preparing a metallic composite material. This technique is widely used as a means of reducing weight in which Al alloys are formed into casting, as referred to Al die-casting. However, in view of the properties of Al itself, the heat resistance etc. is inadequate, and attention is being focused on improvement of the properties of Al by alloying and methods of use for the preparation of metallic composite materials. Similarly, there is a possibility of use for reinforcing the mechanical strength of Mg alloys.
A technique for the preparation of metallic composite materials using metal porous bodies is disclosed in detail in Japanese Patent Application Laid Open No. 9-122887. According to the disclosure in this publication, composited light metal alloys can be used in particular in parts which are subject to use under severe conditions, such as sliding parts for example. Consequently, the metal porous bodies used for the preparation of such composite materials must have properties which satisfy the requirements in the application in which they are to be used.
The aforementioned xe2x80x9cCELMETxe2x80x9d is already being used as a metal porous body for the preparation of metallic composite materials, and a technique which is intended to bring about an especially advantageous effect in properties has been disclosed in Japanese Patent Application Laid Open No. 10-251710. According to the description, the metal porous body is obtained by applying a slurry which contains metal powder and ceramic powder onto a foamed material capable of burning out, then burning out the resin part in a reducing atmosphere where steam and/or carbon dioxide is contained in a reducing gas; and then firing the metal body in a reducing atmosphere. As a result, the ceramic particles are dispersed within a skeleton of the resultant metal porous body and a metal porous body with the properties of ceramic is obtained.
As described above, the techniques of filling the skeletons of metal porous bodies with molten metal for producing metallic composite materials have been progressed day by day for the improvement of the properties of the metallic composite materials.
As for the techniques of metallic composite materials, studies have been made on various techniques for the preparation of composite materials from Al or Mg metal and further for the preparation of composite materials from Al alloys and Mg alloys, and problems which are encountered when using metallic composite materials have been resolved by these studies. Metallic composite alloys are gaining attention and being used as materials for automobile engine parts, for example. However, the requirements for engine materials have become even more severe for the purpose of the improvements which are being made in view of the automobile exhaust gas regulations etc., and further improvement of their properties is now required. For the parts used in wear resistant piston rings in diesel engines in particular, much improved wear resistance is required to the composite materials to be used. There is also a means of compositing by using the metal porous bodies containing ceramic particles, as disclosed in the aforementioned publication, but, when such means is used, the pre-forming process of the ceramic containing metal porous body is difficult and this imposes a limitation on shape.
The present invention has been realized as a result of investigations based upon the demands for such technical improvements, and it provides a material having performance which meets these demands. Specifically, the present invention provides a metal porous body with a foam structure, a skeleton of which is composed of an alloy containing Fe and Cr, and in which a Cr carbide and/or FeCr carbide is uniformly dispersed. The amount of metal carbide contained can be determined from a carbon content, and a carbon content of at least 0.1% but not more than 3.5% in the skeleton of a metal porous body brings about especially desirable properties. The metal porous body principally consists of,Fe and Cr and a Cr carbide and/or FeCr carbide is uniformly dispersed in the composition, which provides the metal porous body with a strength that has never been realized before. It is especially desirable that the carbon content calculated from the amount of carbides be within the range indicated above. If the carbon content is less than 0.1%, then the amount carbide in the skeleton is small and so the wear resistance is poor, and if the carbon content exceeds 3.5% then the skeleton itself becomes hard and difficulties may arise when working a prefoam, as referred to in the same way as previous techniques in which ceramic particles were used. Further, the metal porous body having an excess or insufficient carbon content causes problems such that the metallic composite material prepared from such a metal porous body has poor workability or causes wear of the counterpart when used in a sliding part or device. A carbon content of 0.3 to 2.5% will provide further improved properties.
In the aforementioned preferred carbon content range, the Vicker""s hardness of the skeleton of the metal porous body is within the range from 140 to 350, which brings about a good effect in particular in the workability and wear resistance after being formed into a composite material.
In the present invention, the metal skeleton preferably contains at least element selected from the group consisting of Ni, Cu, Mo, Al, P, B, Si and Ti, so that toughness is increased even more.
The method for preparing a metal porous body in accordance with the present invention is as follows.
A slurry composed mainly of an Fe oxide powder of average particle size not more than 5 xcexcm, at least one powder selected from the group consisting of metallic Cr, Cr alloy and Cr oxide powders, a thermosetting resin and a diluent is prepared, this slurry is applied onto a foamed (porous) resin core and then dried. In the subsequent firing process in a non-oxidizing atmosphere including a heat-treatment at a temperature of 950 to 1350xc2x0 C., a sintered body which has a skeleton composed mainly of the aforementioned Fe and Cr and having a Cr carbide and/or Cr carbide uniformly dispersed is obtained. When this is done, the metal carbide is in a uniformly dispersed state, unlike that in the case where metal carbide is added from the beginning as a carbon component. Further, the metal carbide phase resulting from the process of the present invention has a mean grain size in the range of 2 xcexcm to 5 xcexcm and brings about a good effect in properties such as wear resistance of the resultant metal porous body.
The aforementioned additional metals are included in the skeleton of the alloyed metal porous body after sintering by mixing the metal powder in the slurry.
A preferred embodiment of the aforementioned firing process is characterized by including a first heat-treatment step in which the resin component of the porous resin core on which the slurry has been coated and dried is carbonized in a non-oxidizing atmosphere, and a second heat-treatment step at a temperature from 950xc2x0 C. to 1350xc2x0 C. in a reducing atmosphere in which part of the metal component (the Fe oxide and at least one of Cr, its oxide or alloy) is converted to carbide while the metal oxide is reduced by the carbonized component which has been produced in the first heat-treatment, and then the reduced metal fraction is alloyed and sintered.
In this embodiment, by using Fe, which forms the base of the metal porous body, as finer particles and adding the first heat-treatment step prior to sintering, a metal porous body of an Fe and Cr alloy can be obtained with high strength, heat resistance and corrosion resistance. By production using this method in particular, the resultant metal porous body has a metal density which is increased in the cross section of the metal porous body skeleton and an open pore area ratio of not greater than 30%.
The key factors to be especially noted in the preparation process are the mixing proportion the resin component, which provides the carbon source for forming the carbides, and the firing conditions.
It is preferable that the ratio of the carbonized component, which has been produced in the heat-treatment step from the porous resin porous body and the resin component used in the slurry, and the Fe oxide and other oxide powder which is added to the slurry, be preferably within a certain range, and the formulation of the slurry is best determined on the basis of this relationship. The best method of determining this ratio is that, in the mixing ratio of the resin component, such as the thermosetting resin, to be mixed to be added in the slurry and the oxide powders, the rate of th e carbon residue of the whole resin component, including the resin porous body which may remain in the skeleton of the porous body formed by the heat-treatment step, and the ratio by weight of the whole resin component to the oxide should be within the range which satisfies equation (1) below.
11 less than Xxc3x97Y less than 38xe2x80x83xe2x80x83(1)
where:
X=rate (wt %) of the carbon residue of the resin component; and
Y=ratio by weight of the resin component with respect to the oxide.
The above rate xe2x80x9cXxe2x80x9d of carbon residue of the resin component is the total rate of the carbon residue of the whole resin component, mainly the thermosetting resin added to the slurry and the resin porous body constituting the initial skeleton. The rate of the carbon residue is determined using the method described in JIS K2270. Specifically, a slurry is prepared from the thermosetting resin, dispersing agent, solvent, diluent, etc. (excluding the metal component, such as metal oxide, metal, etc.), applied onto a resin porous body and dried. The whole weight of the thus dried resin component (i.e., the total weight of the resin porous body, thermosetting resin, dispersing agent, etc.) is measured and indicated as W2. Then, the resin porous body on which the thermosetting resin, etc. have been applied is heated and carbonized as specified in JIS K2270 and the resultant weight W3 is measured. The rate xe2x80x9cXxe2x80x9d (wt %) of the carbon residue of the resin component is obtained from the equation W3/W2xc3x97100.
xe2x80x9cYxe2x80x9d is calculated from the equation W2/W4 wherein W2 is the weight of the dried resin component as specified above and W4 is the weight of the metal oxide in the metal component (metal oxide, metal, etc.) to be added to the slurry. The metal oxide is mainly Fe oxide, but, when Cr oxide has been used, this component is included. Under such mixing conditions, the reduction of the oxide proceeds with a good balance in the second step and metal porous bodies which have superior strength can be obtained.
In cases where it is desirable that the carbon content in the metal porous body obtained is between 0.1% and 3.5%, the mixing ratio of the oxide powder and the thermosetting resin is preferably so controlled as to satisfy the following equation (2):
5.1 less than axc3x97b less than 11xe2x80x83xe2x80x83(2)
wherein xe2x80x9caxe2x80x9d is the rate (wt %) of the carbon residue of the thermosetting resin to be added in the form of solution to the slurry and xe2x80x9cbxe2x80x9d is the ratio by weight of the thermosetting resin added in the form of solution to the slurry with respect to the metal oxide. This rate of the carbon residue is calculated fromt eh following equation:
a(wt %)=c:3/c2xc3x97100 and b=c2/c4,
wherein c2 is the weight of the thermosetting resin solution to be added to the slurry, c3 is the weight remaining after heating and carbonizing the thermosetting resin solution, as specified in JIS K2270 and c:4 is the weight of the metal oxide and is the same as the above W4.
Throughout the specification, X and Y in equation (1) and a and b in equation (2) are calculated as described above.
The sintering conditions of the production process of the present invention are also influenced by the carbon source which is contained in the slurry and the amount of oxygen in the metal oxides. Some change must be made to the conditions according to the compounded amounts.
Since the metal porous body formed in this way has a uniformly dispersed metal carbide phase and a metal phase and the metal carbide phase is composed of carbide in every part thereof including the interior part, it has high toughness and superior wear resistance.
The metal porous bodies are suitable for the preparation of metallic composite materials by pouring in an Al alloy or Mg alloy melt. The preferred metallic composite materials are formed, in particular, with the pouring in of an Al alloy or Mg alloy melt under a pressure of at least 98 kPa wherein the Al alloy or Mg alloy matrix conforms with the metal porous body upon compositing.
Moreover, it is possible to obtain alloys suited for the intended use by adding a third material other than alloys of Fe and Cr. That is to say, addition of a third powder or its oxide powder will produce an effect of enhancing the heat resistance, corrosion resistance, wear resistance, mechanical strength, etc. Ni, Cu, Mo, Al, P, B, Si, and Ti are typical examples of such a third material. These third materials may be added as metal powder or oxide powder. Since some materials can be easily obtained as powders when they are in the state of oxide even if they are difficult to form into powders when they are in a state other than oxide. The invention is very useful in such cases as well.
In those cases where the aforementioned third material is added as an oxide, this oxide of the third material is also taken into consideration in xe2x80x9cYxe2x80x9d and xe2x80x9cbxe2x80x9d in the earlier relationship equations (1) and (2), respectively.